1. Field of the Invention
The present invention relates to an optical waveguide device, such as an optical encoder or an optical decoder including an optical waveguide, in which a Bragg diffraction grating is formed.
2. Description of the Related Art
A fiber Bragg grating (FBG) is a Bragg diffraction grating having a periodic refractive index modulation structure formed in an optical fiber core, and is a filter which reflects light of a specific wavelength. The FBG is made from an optical fiber, and is therefore applied to optical communication devices. FBG are mainly manufactured by the phase mask technique and the Amplitude-Splitting Interferometer technique. The phase mask technique has the advantage that FBG with identical characteristics can be produced with sufficient reproducibility and an ultraviolet radiation laser of low coherence can also be used. The Amplitude-Splitting Interferometer technique has the advantage that the period of the Bragg diffraction grating can be freely set by changing the angle at which the ultraviolet radiation beams split into two are made to interfere, but an ultraviolet radiation laser of high coherence is required. See non-patent document 1, i.e., Toru Mizunami, “Optical fiber diffraction gratings”, Applied Physics, Vol. 67, No. 9, 1993, pp. 1029–1033., for example.
An FBG having a phase shift section in a Bragg diffraction grating (SS-FBG: Superstructure-FBG) has also been applied to an OCDM (Optical Code Division Multiplexing) encoder and decoder. See non-patent documents 2, i.e., Akihiko Nishiki et al, “Development of OCDM Phase Encoder Using SSFBG”, Technical Report of IEICE, OFT2002-66, November 2002, pp. 13–18, and non-patent documents 3, i.e., Hideyuki Sotobayashi, “Optical Encoding Division Multiplexing Networks”, Applied Physics, Vol. 71, No. 7, 2002, pp. 853–859, for example. In the OCDM encoder and decoder, plural phase shift sections are formed at desired positions in the refractive index modulation structure of the FBG. In the encoder and decoder, the number and position of phase shift sections in the refractive index modulation structure of the FBG are determined according to the code length and code pattern.
FIG. 24 is a diagram showing the principle of coding by the encoder (hereafter referred to as “FBG encoder”) 310 using SS-FBG and decoding by the decoder (hereafter referred to as “FBG decoder”) 320 using SS-FBG. FIG. 24 shows the construction when the FBG encoder 310 or FBG decoder 320 respectively generate a binary phasing signal of code length ‘4’. The FBG encoder 310 has four FBG units (hereafter referred to as “unit FBG”) 311–314, and the FBG decoder 320 has four unit FBGs 321–324. In the unit FBGs 311–314 and 321–324, the relative phases of the diffraction gratings of the unit FBGs are identical (the amount of phase difference is 0), or the relative phases differ by the amount of phase π. In the FBG encoder 310 shown in FIG. 24, the phase of the unit FBG 313 situated third from the left differs relatively from the phases of the other three unit FBGs 311, 312, and 314 by the amount of phase π. In the FBG decoder 320 shown in FIG. 24, the phase of the unit FBG 322 situated second from the left differs relatively from the phases of the other three unit FBGs 321, 323, and 324 by the amount of phase π.
As shown in FIG. 24, when a light pulse 330 is inputted into the FBG encoder 310, and the reflectance of the unit FBG is small, the positions of each unit FBGs 311–314 differ, so four light pulses (hereafter referred to as an “encoded pulse train”) 341–344 respectively reflected by the unit FBGs 311–314 arise with a time difference. The four light pulses 341–344 respectively reflected by the unit FBGs 311–314 relatively have an optical phase of 0 or π according to the relative phases of the diffraction gratings of the unit FBGs 311–314. Therefore, the four light pulses 341–344 reflected by the unit FBGs 311–314 shown in FIG. 24 relatively have optical phases of (0, 0, π, 0), respectively. When the light pulses 341–344 of this encoded pulse train enter the FBG decoder 320, four light pulses 341a–341d, 342a–342d, 343a–343d, and 344a–344d reflected by the unit FBGs 321–324 corresponding to each of the light pulses 341–344 arise with a time difference as in the FBG encoder 310 (i.e., a total of 16 light pulses 341a–341d, 342a–342d, 343a–343d, and 344a–344d are generated).
In the FBG decoder 320 shown in FIG. 24, the phase of the diffraction grating of the unit FBG 322, which is second from the left, differs relatively from the phases of the other three diffraction gratings of the unit FBGs 321, 323, and 324. In other words, the phase sequence of the diffraction gratings of the unit FBGs 321–324 of the FBG decoder 320, is opposite to the phase sequence of the diffraction gratings of the unit FBGs 311–314 of the FBG encoder 310. At this time, the code pattern of the FBG encoder 310 and the code pattern of the FBG decoder 320 coincide. The light pulse reflected by the unit FBGs 321–324 of the FBG decoder 320 has an optical phase of (0, π, 0, 0) or (π, 0, π, π), respectively. If some of the 16 light pulses 341a–341d, 342a–342d, 343a–343d, and 344a–344d reflected by the FBG decoder 320 are superimposed and the superimposed light pulses are identical, they reinforce each other, and if the phases are different, they cancel each other out. As shown in FIG. 24, when the code pattern of the FBG encoder 310 and the code pattern of the FBG decoder 320 is the same, the four light pulses 341d, 342c, 343b, and 344a are superimposed and match each other, while at other times, light pulses of the same phase and different phase are superimposed and cancel out, so a light pulse 353 having a high resolution peak is produced at a specific time.
FIGS. 25A and 25B are diagrams showing a refractive index modulation structure 304 of a core 302 of an optical fiber 301 with which an FBG encoder (or FBG decoder) 300 is provided. A diffraction grating formed in the optical fiber core 302 has a periodic refractive index modulation Δn. For a desired reflected wavelength λ, the diffraction grating period Λ is given by the Bragg reflection equation:λ=2×neff×Λ  (1)where neff is the effective refractive index of the optical fiber.
The refractive index modulation Δn of the FBG encoder 300 shown in FIGS. 25A and 25B has a structure wherein plural phase shift sections 306 are provided depending on the code pattern. The FBG encoder 300 shown in FIGS. 25A and 25B has seven phase shift sections 306.
However, in conventional FBG encoders and FBG decoders, if an input light pulse width is made larger than a time difference (hereafter referred to as a “reflected pulse time difference”) of the reflected pulse between adjacent unit FBGs, the reflected pulses of the adjacent unit FBGs will be superimposed, and interference will be caused between the reflected pulses. If the distance between the unit FBGs is ‘d’, the reflected pulse time difference Td is given by the equation:Td=2×d×neff/c  (2)where ‘c’ is the velocity of light in a vacuum.
FIGS. 26A to 26D are diagrams showing that interference arises between the reflected pulses of the adjacent unit FBGs when the input light pulse width is larger than the reflected pulse time difference. As shown in FIG. 26A, when a light pulse 360 having a larger pulse width than the reflected pulse time difference Td of unit FBGs 371 and 372 is inputted into an FBG 370 including the two unit FBGs 371 and 372, a light pulse 372a reflected by the unit FBG 372 adjacent to a light pulse 371a reflected by the unit FBG 371, has a reflected pulse time difference, but as shown in FIG. 26B, parts of the two reflected pulses 371a and 372a overlap. When the reflected pulses 371a and 372a have a different phase, a distortion of the light pulse shown in FIG. 26C occurs, and if they have the same phase, the light pulses combine as shown in FIG. 26D. In other words, in the conventional FBG encoder, when the input light pulse width is larger than the reflected pulse time difference Td, the generated encoded pulse train collapses.
Further, FIG. 27 is a diagram showing a graph of the relation between the input light pulse width and the bandwidth of a unit FBG, and the relation between the reflected pulse time difference and the bandwidth, when the input light pulse width is smaller than the reflected pulse time difference Td. As shown in the graph of FIG. 27, when the input light pulse width is smaller than the reflected pulse time difference, the input light pulse bandwidth is larger than the filter bandwidth of the unit FBG. The reflected light pulse width due to the unit FBG therefore spreads out, the reflected pulses of adjacent unit FBGs overlap, interference between light pulses occurs, and in the FBG encoder, the encoded pulse train collapses.
Therefore, whether the input light pulse width is larger or smaller than the reflected pulse time difference, interference arises between the light pulses of the encoded pulse train generated by the conventional optical encoder, and the reproducibility of the light pulses decoded from the conventional FBG decoder deteriorates.